Almost passed out .. 34week

So i was stood in the chinese takeaway waiting to collect dinner with my 2 children and all of a sudden i just went overly warm and my vision went i realised i was stumbling and immediately crouched before i fell.. Noone offered me help or even asked if i was okay which really pissed me off but hey ho never mind.. What could of caused this tho and is there anything i can do to avoid it happening again? It was really scary!!